GPU rendering for Cycles on MAC PRO (late 2013)
The original graphics cards that came with the mac pro's aren't up to much so I'd expect the cpu to render faster. However if like me you want to stick with the mac pro you can get an nvidia card and use their web drivers. 

I think there are many PC cards you can put in there (you'll just lose the boot screen). I got a GTX 980 from Mac Vid Cards which is awesome to render on. Sometimes I get a CUDA error but generally a restart of blender fixes that.

Another option that I use is Ray Pump. You need to pay but it's really cheap. They've been down for maintenance but I assume they'll be back up soon.